1. Field of the Invention
Embodiments disclosed herein relate generally to polycrystalline composites used in cutting tools.
2. Background Art
Historically, there have been two types of drill bits used for drilling earth formations, drag bits and roller cone bits. Roller cone bits include one or more roller cones rotatably mounted to the bit body. These roller cones have a plurality of cutting elements attached thereto that crush, gouge, and scrape rock at the bottom of a hole being drilled. Several types of roller cone drill bits are available for drilling wellbores through earth formations, including insert bits (e.g. tungsten carbide insert bit, TCI) and “milled tooth” bits. The bit bodies and roller cones of roller cone bits are conventionally made of steel. In a milled tooth bit, the cutting elements or teeth are steel and conventionally integrally formed with the cone. In an insert or TCI bit, the cutting elements or inserts are conventionally formed from tungsten carbide, and may optionally include a diamond enhanced tip thereon.
The term “drag bits” refers to those rotary drill bits with no moving elements. Drag bits are often used to drill a variety of rock formations. Drag bits include those having cutting elements or cutters attached to the bit body, which may be a steel bit body or a matrix or composite bit body formed from a matrix material such as tungsten carbide surrounded by an binder material. The cutters may be formed having a substrate or support stud made of carbide, for example tungsten carbide, and an ultra hard cutting surface layer or “table” made of a polycrystalline diamond material or a polycrystalline boron nitride material deposited onto or otherwise bonded to the substrate at an interface surface.
Polycrystalline diamond (PCD), a composite material formed from diamond particles, comprises a polycrystalline mass of diamonds (typically synthetic) that are bonded together to form an integral, tough, high-strength mass or lattice. A metal catalyst, such as cobalt, may be used to promote recrystallization of the diamond particles and formation of the lattice structure. Thus, cobalt particles are typically found within the interstitial spaces in the diamond lattice structure. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired. However, cobalt has a significantly different coefficient of thermal expansion as compared to diamond. Therefore, upon heating of a diamond table, the cobalt and the diamond lattice will expand at different rates, causing cracks to form in the lattice structure and resulting in deterioration of the diamond table. Additionally, the presence of cobalt in the interstitial spaces may catalyze the graphitization of the diamond upon exposure to frictional heat generated during cutting.
In order to obviate this problem, strong acids may be used to “leach” the cobalt from the diamond lattice structure. Examples of “leaching” processes can be found, for example in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a hot strong acid, e.g., nitric acid, hydrofluoric acid, hydrochloric acid, or perchloric acid, or combinations of several strong acids may be used to treat the diamond table, removing at least a portion of the catalyst from the PCD layer. By leaching out the cobalt, thermally stable polycrystalline (TSP) diamond may be formed. In certain embodiments, only a select portion of a diamond composite is leached, in order to gain thermal stability without losing impact resistance.
However, several problems exist with the leaching cobalt from polycrystalline diamond. Firstly, leaching is performed with strong acids during which the strong acids may attack the surfaces of the diamond grains within the polycrystalline diamond and cause micro-pitting (notches) of the diamond grain. Such micro-pits are weak points that degrade the diamond grain and hence the ability of the diamond grain to support a load, thus giving the structure a susceptibility to crack origination. Additionally, interstitial voids in the polycrystalline diamond are left following the removal of cobalt, leaving the structure weakened to cutting loads, brittle, and less shock resistant. Further, leaching frequently results in non-uniform removal of cobalt, leaving pockets of non-leached cobalt within the diamond lattice, which can retain heat and cause localized graphitization of diamond, limiting the life of the cutter.
Alternatively, TSP may be formed by forming the diamond layer in a press using a binder other than cobalt, one such as silicon, which has a coefficient of thermal expansion more similar to that of diamond than cobalt has. During the manufacturing process, a large portion, 80 to 100 volume percent, of the silicon reacts with the diamond lattice to form silicon carbide which also has a thermal expansion similar to diamond. The thermal resistance is somewhat improved, but thermal degradation still occurs due to some residual silicon remaining, generally uniformly distributed in the interstices of the interstitial matrix. Further, there are mounting problems with this type of PCD element because there is no bondable surface.
Generally, such conventional PCD materials exhibit extremely high hardness, high modulus, and high compressive strength, and provide a high degree of wear protection to a cutting or drilling element. However, in more complex wear environments known to cause impact and fretting fatigue, layers formed from conventional PCD can fail by gross chipping and spalling. For example, drilling inserts coated with a thick PCD monolayer may exhibit brittleness that causes substantial problems in practical applications. Conventional methods of improving the performance of PCD layers include optimizing grain size and controlling cobalt content to increase toughness, but the effect of these methods is limited. Further, attempts to improve the thermal operating limitations of the diamond to greater than that of conventional PCD (˜750° C.), while somewhat effective with respect to thermal instability, frequently exchange one problem (thermal instability) for another, as described above.
Designers continue to seek improved properties (such as improved wear resistance, thermal resistance, fracture toughness, etc.) in the ceramic materials. Further, as the bulk particles used in ceramic materials decrease in size with the increasing use of nanoparticles (grain sizes less than 100 nm), observed brittleness has limited potential applications for the resulting material. It has been known for some time that the addition of fibrous materials to materials may increase mechanical properties, such as strength. However, incorporation of the fibrous materials, such as carbon fibers, has presented difficulties including resistance to wetting of the fibers.
Accordingly, there exists a need for improvements in the material properties of diamond composite materials used in drilling applications.